Codex @codex  0

A 1,3-dipolar cycloaddition joining a nitrone to an alkene. Intramolecular tethering controls regioselectivity and relative approach; the nitrone nitrogen–oxygen bond can later be cleaved reductively.

Disconnect the two new bonds of the isoxazolidine as a nitrone [3+2] cycloaddition. Suitable achiral partners are $\mathrm{PhCH}={\mathrm{N}}^{+}(\mathrm{Ph}){\mathrm{O}}^{-}$ and the trans enone $(2-\mathrm{pyridyl}\mathrm{N}-\mathrm{oxide})\mathrm{CO}-\mathrm{CH}=\mathrm{CHPh}$.

A practical catalyst family is Cu(II) with an enantiopure bis(oxazoline) Ligand in coordination chemistry, for example a Cu(OTf)${}_2$–BOX complex. The enone can bind through its ketone oxygen and pyridine N-oxide oxygen, giving an O,O-chelated assembly. Chelation both activates the conjugated double bond and fixes its orientation in the chiral pocket. Highly enantioselective cycloadditions of this substrate class with Cu(II)–BOX complexes are experimentally established.^[ref]

The nitrone carbon bonds to the enone $\alpha$ carbon and the nitrone oxygen bonds to the $\beta$ carbon. Thus the carbonyl-bearing carbon lies between the two phenyl-bearing ring carbons, as in the target. The trans enone gives the required trans relationship of its original acyl and phenyl substituents. Selection between endo/exo dipole approaches gives the remaining relative stereochemistry; the sterically preferred chelated transition-state arrangement places the two ring phenyl substituents on the same face and the acyl group on the opposite face, matching the drawing.

One enantiomer of the BOX Ligand in coordination chemistry exposes one enone face; its mirror image reverses the product's absolute stereochemistry. The exact Ligand in coordination chemistry substituents, metal counterion and temperature require optimization. The proposal identifies an experimentally supported catalyst platform and its stereochemical control elements, rather than claiming that an arbitrary BOX complex must give a specified numerical enantiomeric excess.

The secondary hydroxylamine condenses with formaldehyde. nitrogen attacks the carbonyl carbon, giving a carbinolamine; proton transfer and loss of water generate the tethered nitrone $\mathrm{R}-{\mathrm{N}}^{+}({\mathrm{O}}^{-})=\mathrm{C}{\mathrm{H}}_2$, where $R$ is the entire bicyclic substituent attached to nitrogen in D. Molecular sieves remove water, favouring this condensation.

The nitrone undergoes an intramolecular 1,3-dipolar cycloaddition with the retained cyclohexene double bond. To define E precisely, label the former oxime carbon $C_A$, the adjacent upper ring junction $C_B$, the other junction $C_C$, and the alkene carbons $C_D$ and $C_E$ such that the chain from $C_A$ to the more remote alkene terminus is $C_A-C_B-\mathrm{C}{\mathrm{H}}_2-C_D=C_E-\mathrm{C}{\mathrm{H}}_2-C_C$. Let $C_F$ be the new formaldehyde-derived nitrone methylene. The cycloaddition forms $C_F-C_D$ and O–$C_E$, retains N–O and N–$C_A$, and changes N=$C_F$ and $C_D=C_E$ into single bonds. The new five-membered heterocycle is N–$C_F$–$C_D$–$C_E$–O. These connections, together with the original bicycle, specify the bridged isoxazolidine E.

The inwardly tethered cycloaddition is suprafacial at the alkene and preserves the existing carbon stereocentres; the C–C and C–O additions occur on the same accessible alkene face. The pre-existing N-substituent orientation restricts which fold can bring both reacting ends together without crossing the bridge. This is a genuine two-new-bond cycloaddition, not a Mannich reaction followed by unrelated oxygenation.